The present invention relates to the transmission of information over a communication link, such as a telephone line, and to the encoding and decoding of data and to the storage of such encoded information. This application is a continuation in part of U.S. patent application Ser. No. 08/254,164, filed Jun. 6, 1994 for PROCESS AND APPARATUS FOR TRANSMITTING VIDEO SIGNALS OVER A COMMUNICATION LINK.
Common or ordinary voice grade telephone lines have been utilized for a number of years in connection with the transmission and reception of signals, other than audio signals. Common or ordinary voice grade telephone lines are telephone lines that have substantially the same predetermined or standard bandwidth, i.e. about 0-3600 Hz, that make up the substantial majority of telephone lines in the United States, as well as in foreign countries, for providing the telephone linkage among residences, public telephones and most businesses. Ordinary telephone lines have also been used to transmit video signals. The ordinary telephone line, having a bandwidth of about 0-3600 Hz or a transmission rate of about 9.6 kbaud, does not transmit, in real time, a typical full motion commercial television type black and white and/or color video image. The commercial television system displays 512.times.512 pixel images at 30 frames per second and uses about 6 MHz bandwidth while simultaneously transmitting video and audio signals. Because of the large bandwidth required, prior art systems do not enable one to transmit full motion images over an ordinary voice grade telephone line. In connection with the transmission of video and audio signals, the video signal is transmitted over the ordinary telephone line using a first, predetermined bandwidth of the limited bandwidth of the ordinary telephone line and the audio signal is transmitted using a second, predetermined bandwidth of the limited bandwidth of the ordinary telephone line. U.S. Pat. No. 4,849,811 to Kleinerman, issued Jul. 18, 1989, and entitled "Simultaneous Audio and Video Transmission with Restricted Bandwidth" describes a system in which modulated digitized image signals and filtered voice signals are transmitted together over an ordinary telephone line whereby still or freeze-frame images are provided with accompanying video. The digitized image signals are in the range of 2400 to less than about 400 Hz. A low pass filter limits the voice signals to a range outside the digitized image signals so that the image signals and voice signals can be transmitted at the same time but over separated sections of the limited bandwidth of the telephone line. Because of the separate bandwidths used, means must be provided for synchronizing the sending and/or receiving of the video and audio signals at the received end. In conjunction with the more rapid transmission of video images, the use of known data compression techniques is mentioned in this patent. Similarly, in U.S. Pat. No. 3,873,771 to Kleinerman, issued Mar. 25, 1975, and entitled "Simultaneous Transmission of a Video and an Audio Signal Through an Ordinary Telephone Transmission Line", a communication system is disclosed for transmitting video and audio information using separate bandwidths of the limited bandwidth of an ordinary telephone line. With regard to the transmission of video information, it is accomplished using slow scan TV techniques so that an image is not transmitted in real time, but rather the transmission requires up to about 8 seconds to transmit an image with 120 scan lines per image.
In another technique for transmitting video and audio signals, two signals are multiplexed in such a way to enable one of the two signals to be sent when the other of the two signals is not being transmitted.
These prior art systems are not capable of transmitting, in real time, the audio and the moving video image data together over an ordinary voice grade telephone line. Such prior systems require from about 3-60 seconds to transmit a still image. This occurs because voice grade telephone lines typically have a bandwidth of only about 0-3600 Hz. Because of this limited bandwidth, the amount of data or information that can be transmitted in a given time is limited. To overcome this drawback, it is known to use transmission lines, other than ordinary telephone lines, for transmitting voice and video data. In such systems, transmission lines are used having a significantly greater bandwidth than that of ordinary telephone lines, such as fiber optic lines. With regard to fiber optic transmission lines or other transmission lines having a much greater bandwidth than the ordinary telephone line, it is known to transmit video and audio signals in real time.
Simultaneous transmission of three television signals is disclosed in U.S. Pat. No. 4,593,318 to Eng et al., issued Jun. 3, 1986 and entitled "Technique for the Time Compression Multiplexing of Three Television Signals". In one embodiment of the system, a time compression multiplexing technique enables the transmission of three color television signals through a satellite transponder having a 36 MHz bandwidth in which one field signal and two field differential signals are each time compressed to permit all three signals to be sent in the period of a normal field signal of a standard TV signal. Since there are three TV sources, with each producing stereo audio, six audio signals are also transmitted. The stereo audio from each source is sent along with the video by inserting digital audio in either the vertical or horizontal blanking periods associated with the video.
In addition to providing an increased bandwidth in order to transmit a plurality of signals including video and audio signals, as some of the prior art indicates, data compression techniques are employed so that compressed video information can be transmitted for subsequent expansion at a receiver station, without meaningful loss of transmitted information. Various compression methods have been suggested for transmitting video image data over a digital telephone line. To achieve the compression, spectral, spatial and temporal compression techniques are employed. These data compression techniques are utilized in such a way to exploit the human eye's forgiving nature so as to make the tradeoffs that cause the least objectionable losses in picture quality. In connection with the compression, comparisons are made between new pixel information and previously transmitted pixel information so that only video information that is changing need be sent.
Other U.S. patents that use telephone lines for the transmission of video data can be seen in U.S. Pat. No. 5,164,980 for a video telephone and in U.S. Pat. No. 5,202,951 for a Full Motion Video Telephone System. U.S. Pat. No. 3,702,899 for a Digital Video Transmitter uses a conventional telephone system. U.S. Pat. No. 3,974,329 is for the Transmission of Video Pictures at audio frequencies. Two U.S. patents to Yurt et al. U.S. Pat. No. 5,253,275 and U.S. Pat. No. 5,132,992 each teach an audio and video transmission and receiving system. U.S. Pat. No. 4,620,217 for a standard transmission and recording of high resolution television separates the modulation frequency as sidebands with pixel information out from the video signal from the television camera. A heterodyne converter is used to convert the sidebands down to a color subcarrier band.
The Gitlin et al patent, U.S. Pat. No. 4,924,492, is for a method and apparatus for wide band transmission of digital signals between, for example, a telephone central office and customer premises and uses a telephone local loop transmission arrangement in which data is communicated from the customer's premises to a central office utilizing a multi-dimensional pass-band signal. In the U.K. patent application, GB 2,173,675A, a communication system for sending video and audio signals to a telephone line supplies a digitized video signal from a camera and uses an analog-to-digital converter along with a frame storage and a parallel-to-series converter. The signal bytes from the converter and sync signals from a timing generator are supplied to a frequency shift keyer to produce an output signal at any one of several distinct frequencies within the audio frequency band of the telephone line.
In summary, many systems have been proposed or devised for transmitting video information and/or audio information over telephone lines but none has been provided that relatively inexpensively sends and receives, in substantially real time, both video information and audio information over an ordinary voice grade telephone line. It would be advantageous to have such a system in order to provide real time viewing.
In contrast to the prior art, the present invention allows the transmission of real time video signals over twisted pair copper telephone lines in which monochrome composite color signals are phase modulated relative to a fixed frequency reference tone in the audio frequency range to form a narrow passband, such as 100 Hz. The phase modulated signals are phase detected between the reference tone and the modulated video signal tones and have been converted to a constantly varying phase modulated signal in an audio frequency spaced from the reference tone by a small DC voltage, i.e. about 0.17 volts. The system is relatively inexpensive and sends and receives, in real time, both video information and audio information over ordinary voice grade twisted pair telephone lines and is transmitted through all types of lines including fiber optic lines and through electrical switches and repeaters. The present invention is further directed to increasing the rate at which data can pass through a transmission medium so as to increase the distance over which high bandwidth signals can be transmitted in a low bandwidth medium. This increase of the apparent bandwidths of a transmission medium include low bandwidth telephone lines and cable as well as fiber optic lines and broadcast over the airwaves as well as a communication satellite link. In one specific embodiment of the invention, monochrome or RGB color signals are transmitted over a low bandwidth telephone line or other communication link over substantial distances with a minimum degradation thereof. The transmission of the video signals can be over conventional telephone lines through an existing telephone system including an international communication link over large distances, such as several thousand miles, with no modification of the telephone system or line and with only partial degradation of the picture quality. In addition, the video signals can be passed through the conventional switching equipment of telephone exchanges and through the conventional repeaters used in telephone lines without rejection as high frequency noise. The present invention also provides an improved encoder and decoder for the transmission of stored information including a variable reference line for referencing phase modulated data which is phase detected through operational amplifiers or the like to provide a high sensitivity to the predetermined frequency and which detectors can operate to drive a voltage controlled frequency converter circuit to produce frequency input to the encoder. Another improvement in the present invention includes the transmission of high bandwidth signals over a long distance in a low bandwidth transmission medium utilizing a very low signal level in an area below the level of the telephone filters that reduce noise on the telephone line. It is also a purpose of the present invention to be able to use the encoded data placed directly on a storage medium, such as an optical disc storage or magnetic storage, such that large amounts of data are stored on the storage medium in the encoded format which can be decoded by a decoder.
It will be understood by those skilled in the art that a communication link or data channel is a path for transmission between two or more stations or terminals and it can be a single pair of wires or a group of wires in a cable, a coaxial cable, or a fiber optic cable, or a special band of the RF spectrum. The system advantageously can be used interactively in real time. Digital data is generally used in place of analog information in electronic and computer applications so that data communications is the electronic transmission of encoded information or data from one point to another. The present invention utilizes phase modulation. Phase modulation is the phase of the carrier wave is varied by an amount proportional to the amplitude of the message signal. In phase modulation, the instantaneous phase of the carrier is shifted in accordance with the modulating waveform. The extent of the phase shift is directly proportional to the amplitude of the modulated signal. The rapidity of the phase shift is directly proportional to the change in the amplitude and frequency of the modulating signal.